by Gary Strauss, Kevin Johnson and Chris Woodyard , USA TODAY

by Gary Strauss, Kevin Johnson and Chris Woodyard , USA TODAY

LOS ANGELES - Paul Ciancia, the alleged gunman who paralyzed much of Los Angeles International Airport in a Friday shooting spree, could have turned the nation's third-busiest airport into a massive killing zone had it not been for the quick response by airport police, officials said Saturday.

In a criminal complaint, U.S. District Attorney Andre Birotte charged Ciancia, 23, with murdering a federal officer and a separate charge of committing violence at an international airport. An affidavit said Ciancia - who could face the death penalty - fired a .223-caliber assault rifle at point-blank range, killing Transportation Security Agency officer Gerardo Hernandez, wounding two other TSA officers and two civilians.

FBI Special Agent David Bowdich officials said Ciancia walked away from Hernandez after shooting him in a pre-screening area at the airport, then went up an escalator in Terminal 3 before returning and shooting again.

As FBI and Los Angeles police wrapped forensics work, Bowdich, Los Angeles City Councilman Mike Bonin and LAX Police Chief Patrick Gannon said the victim toll could have been much higher had LAX police not shot and wounded Ciancia within minutes, thwarting him from firing more than a few rounds before he was shot four times.

"The heroics of the officers cannot be understated,'' said Bowdich, who noted that the suspect was more than 100 rounds of ammunition.

"They didn't hesitate for one moment,'' Gannon said. "To say it wasn't a traumatic incident for them, I'd be lying, but they handled it with the utmost professionalism."

Gannon said more than 200 LAX and LAPD personnel sharpened crisis skills last month during a training exercise in an abandoned terminal at Ontario International Airport, 40 miles east of L.A. With recent mass shootings such as those in Washington's Navy Yard, Gannon says his force was motivated. "It doesn't take much to want to handle an incident if it occurs in your own backyard,'' he said.

A federal law enforcement official said that the Smith & Wesson assault rifle used in the attack is believed to have been purchased legally from a Los Angeles-area arms dealer. The official also said authorities were investigating whether a roommate unwittingly drove Ciancia to the airport the morning of the attack.

Investigators recovered a rambling note from the bag the shooter allegedly was carrying, which detailed an intent to "kill" TSA officers, said two federal law enforcement officials familiar with the message's contents.

Bowdich said the handwritten note made it clear that the suspect intended to kill "multiple" TSA employees and to "instill fear into their traitorous minds."

The officials, who are not authorized to comment publicly, told USA TODAY that the note was written in a way that suggested the author expected to lose his life.

One of the officials described the incident as a suicide mission.

In recent days, one of the officials said, Ciancia's New Jersey family had become worried about his emotional state and called local police, who relayed their concerns to Los Angeles authorities. The official said LAPD reportedly were in contact with the alleged shooter's roommates, who indicated that he appeared to be OK.

Terrified LAX witnesses said the suspect specifically asked them whether they worked for the TSA.

Leon Saryan told ABC News that the gunman approached him and pointed a long-barreled weapon at him.

"I was cowering in a corner, he looked at me and he said, 'TSA?' I shook my head no, and he kept on going," Leon Saryan said. "I just prayed to God. That's all I did. I just prayed."

Hernandez, 39, was a behavior-detection officer tasked with spotting suspicious activity and identifying potential terrorists, TSA Administrator John Pistole said. Pistole was scheduled to meet with Hernadez' family Saturday.

Hernandez is the first TSA officer killed in the line of duty in the 12-year history of the agency, created after 9/11. Friends and neighbors remembered Hernandez as a doting father of two and a good neighbor.

Before arriving in the L.A. area about 18 months ago, Ciancia lived with family in a quiet, wooded neighborhood in Pennsville, N.J., police said.

Joshua Pagan, 17, has lived across the street from Ciancia's residence for 10 years. He said Ciancia has a brother close in age to him.

"I've seen (Paul) a few times, but I did not know him personally," Pagan said. "From what I've seen and heard, he was just a normal person - just an everyday guy. Right now, I am still trying to process this. Did this really happen? Did they get the wrong guy? Because if they told me they got the wrong guy, it would make a lot more sense to me."

Monitored in real time by scores of terrified bystanders on social media websites such as Twitter and Instagram, the incident was the latest in a recent spate of mass shootings. Some passengers built walls of luggage to protect themselves from flying bullets. Others fled through emergency doors and onto the tarmac, including some who were briefly handcuffed and questioned by swarming groups of police before they were released.

"They probably thought I was the shooter. They handcuffed me and told me not to move," said Nick Pugh, who was scheduled to fly east to watch Sunday's New York Marathon.

Aleksander Fiksdal, 22, was in a security checkpoint line for a flight to his native Norway when he heard shots ring out. "I turned and saw a guy with a rifle and I threw myself on the ground," Fiksdal said. Other passengers in the security line did the same, while some broke and ran.

"I really thought I saw death," said Anne Rainer, who witnessed the gunfire with her 26-year-old son, Ben. The pair were about to leave for New York so her son could see a specialist for a rare genetic condition he has.

They took refuge behind a ticket counter where she said people prayed, cried and held hands. She watched as one person jumped from a second-floor balcony to get away from the gunman.

"Adrenaline went through my head, my body went numb, and I said, 'If I have to go, it's OK because I'm not going to feel it, but I have to save him,'" Rainer said.

Friends Mara Allen, 42, of Yuba City, Calif., and Vicki Powell of Sacramento had just arrived on a flight so they could go on a cruise to celebrate Powell's 50th birthday. Gathered with others around a baggage carousel, they were startled when a police officer started yelling for people to flee. Wheelchair-bound Powell wasn't sure how she was going to get out.

"I was waving. Come get me. Help!" Powell caught the attention of an attendant who wheeled her outside. The pair had to leave their luggage behind, and it was unclear whether they'd be able to retrieve it before their ship leaves L.A. for Mexico. "This is something I see on TV. I don't want to be in it," Powell said.

The last shooting at the airport occurred in July 2002, when an Egyptian immigrant shot and killed two bystanders in a rampage at the El Al ticket counter.